Process for thermal treatment of glass fiber preform

ABSTRACT

The present invention provides 
     a process for the dehydrating and purifying treatment by heating a porous glass preform for an optical fiber by passing the porous glass preform through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere containing an inert gas and a silicon halogenide gas; 
     a process for the fluorine-doping treatment by heating a porous glass preform for an optical fiber by passing a porous glass preform through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere containing a fluorine compound gas and an inert gas; and 
     a process for the vitrifying treatment by heating a porous glass preform for an opticla fiber by passing the preform, which has been previously dehydrated and purified, through a muffle tube having a SiC layer at least on its inner surface at a high temperature under an atmosphere gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the thermal treatment ofa porous glass preform for the production of an optical fiber preform.

2. Description of the Related Art

A preform for an optical fiber is generally produced by forming acylindrical or an annular porous preform by the Vapor Phase AxialDeposition method or the Outside Chemical Vapor Deposition method andheating, dehydrating and sintering the porous preform in a heatingfurnace under an atmosphere of an inert gas such as argon or helium, achlorine based gas or a fluorine based gas to produce a transparent andhighly pure preform for the optical fiber.

An operating temperature of the furnace depends on a kind of a dopant tobe doped and a content of the dopant in the preform, and it is generallyin the range of 1200° to 1600° C. Since impurities tend to contaminatein the preform at such high temperature, a muffle tube made of a highpurity quartz is typically used in the furnace to prevent thecontamination. However, the quartz made muffle tube tends to rapidly getbrittle due to devitrification, that is, transition from a glass phaseto a crystal phase at a high temperature, and thus it is poor indurability.

Recently, a carbon made muffle tube an inner surface of which is coatedwith silicon carbide (SiC) is often used in a heating furnace. Thecarbon made muffle tube can be operated at a higher temperature than thequartz made one and SiC improves a gas impermeability and oxidationresistance of the muffle tube.

Further, in order to prevent a reaction between SiC and a reactive gas,the SiC coating on the carbon made muffle tube is treated with oxygen toform a SiO₂ layer on the SiC coating (see Japanese Patent KokaiPublication No. 201634/1986).

The prior art as described above have following problems:

(1) When the quartz made muffle tube is used, it is softened anddeformed at a temperature above 1400° C. In addition, the temperature ofthe muffle tube cannot be lowered to a temperature below a crystaltransition point (lower than 300° C.) because of crystal (cristobalite)formation at a temperature above 1200° C. Thus, once the muffle tube isheated, it should be used continuously without lowering the temperaturethereof.

(2) When a SiC made or SiC coated muffle tube is used, SiC reacts with ahalogen based gas as a reactant to form a porous carbon, whereby themuffle tube becomes poor in gas tightness so that the halogen based gasleaks outside from the muffle tube.

(3) In the case where the muffle tube having the SiO₂ layer on the SiCcoating is used, thermal expansion coefficients of SiC and SiO₂ aregreatly different from each other and thereby the SiO₂ layer in athickness in the order of some microns tends to crack. Then, a gaspermeates through the crack and it is impossible to stably produce thepreform for a long period.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process of thethermal treatment for the stable production of an optical fiber preformfor a long period from which preform an optical fiber having a lowtransmission loss can be drawn.

The present invention provides a process of the thermal treatment of aporous glass preform for an optical fiber comprising heating the preformin a heating apparatus which comprises a muffle tube having a SiCcoating at least on an inner surface thereof in an atmosphere foreffecting the thermal treatment.

The term "thermal treatment" is intended to mean any treatment in whichthe preform is heated. For example, a sintering treatment, a dehydratingand purifying treatment, a fluorine-doping treatment and a vitrifyingtreatment of the preform are included in such thermal treatment.

In a first aspect, the present invention provides a process for thedehydrating and purifying treatment by heating a porous glass preformfor an optical fiber comprising supporting or passing the preform in orthrough a muffle tube having a SiC layer at least on its inner surfaceat a high temperature under an atmosphere comprising an inert gas and asilicon halogenide gas or an atmosphere comprising an inert gas, asilicon halogenide gas and a halogen based gas.

In a second aspect, the present invention provides a process for thefluorine-doping treatment by heating a porous glass preform for anoptical fiber comprising supporting or passing the preform in or througha muffle tube having a SiC layer at least on its inner surface at a hightemperature under an atmosphere comprising a fluorine compound gas andan inert gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 and 7 each schematically shows a sectional view of anheating furnace in which the present invention is carried out,

FIG. 5 schematically shows an apparatus for measuring an oxygenconcentration in a muffle tube,

FIG. 6 shows results of oxygen measurements in one muffle tube with themeasuring apparatus of FIG. 5, and

FIG. 8 shows a transmission loss of an optical fibers produced inExample 1 and Comparative Example 2.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will be described in detail with reference to theaccompanying drawings.

Now, the first aspect of the present invention is explained.

FIG. 1 schematically shows a sectional view of one embodiment of aheating apparatus in which the process of the present invention iscarried out. The apparatus comprises a muffle tube 12 in which a porousglass preform 11 is inserted and a furnace body 15 around the muffletube 12 comprising a heater 13 to heat the glass preform 11 and aninsulation 14 to prevent heat loss. A dehydrating and purifying gas usedfor dehydrating and purifying the preform is supplied through a line 16connected with the muffle tube. In addition, a silicon halogenide gasgenerated in a bubbler 17 is mixed with the other dehydrating andpurifying gas G in a gas mixer 18. The muffle tube is made of carbon andcoated with a SiC layer. Further, the SiC layer is coated with a SiO₂layer. Each layer can be formed in a conventional manner.

The term "dehydrating and purifying gas" is intended to mean a gasmixture of an inert gas such as helium (He), Argon (Ar) and nitrogen(N₂) with a silicon halogenide gas and optionally a halogen based gas.

The present invention has been conceived on the basis of the followingexperiments:

EXPERIMENT 1

A carbon made plate coated with SiC in a thickness of 100 μm was heatedat 1500° C. under an atmosphere of the dehydrating and purifying gascontaining 2.5 parts by volume of SiCl₄ relative to 100 parts by volumeof the inert gas (He) for 10 hours.

After heating, a weight loss of the SiC coating was determined to be assmall as 1%. A specific surface area of the SiC coating was 0.1 m² /g,which was substantially equal to that of the SiC coating before heating.

EXPERIMENT 2

Experiment 1 was repeated except that the dehydrating and purifying gascontained 3 parts by volume of SiCl₄ and 1 part by volume of Cl₂ eachrelative to 100 parts by volume of the inert gas.

After heating, the weight loss of the SiC coating was determined to beas small as 1.4%. The specific surface area of the SiC coating was 0.15m² /g, which was substantially equal to that of the SiC coating beforeheating.

EXPERIMENT 3

Experiment 1 was repeated except that the dehydrating and purifying gascontained 5 parts by volume of Cl₂ relative to 100 parts by volume ofthe inert gas.

After heating, the weight loss of the SiC coating was determined to beas large as 71%. This means that SiC was completely decomposed. Thespecific surface area of SiC was 800 m² /g and large pores having adiameter of 30 Å were found.

From the above Experiments, the following results can be induced:

(1) At a high temperature, SiC reacts with Cl₂ and does hardly withSiC₄.

(2) The reaction between SiC and Cl₂ can be suppressed when SiCl₄ isadded to Cl₂ in the dehydrating and purifying gas.

Thus, in order to prevent the degradation of the SiC coating due to Cl₂,the dehydrating and purifying treatment is preferably carried out underan atmosphere of SiCl₄ or an atmosphere of Cl₂ mixed with SiCl₄. Thoseconsiderations can be expressed according to the following equilibriumequation:

    SiC+2Cl.sub.2 ←→SiCl.sub.4 +C                  (1)

The above equation indicates the degradation of SiC by Cl₂. Therefore,in order to suppress the degradation, a concentration of the productSiCl₄ is increased on the basis of an equilibrium relation expressed byan equilibrium constant Kp=[C_(SiCl).sbsb.4 ]^(1/2) /[Cl₂ ].

When a gas mixture of the inert gas and the silicon halogenide gas isused as the dehydrating and purifying gas, 0.3 to 10 parts, preferably 2to 5 parts by volume of the halogenated gas is used relative to 100parts by volume of the inert gas. When a ratio of the silicon halogenidegas is less than 0.3 parts by volume, a dehydrating ability of the gasmixture is insufficient. When it is more than 10 parts by volume,increase of the effect due to addition of such amount of SiCl₄ is not soremarkable.

When a gas mixture of the inert gas, the silicon halogenide and thehalogen based gas is used as the dehydrating and purifying gas, 0.3 to10 parts by volume of the silicon halogenide gas and 0.1 to 10 parts byvolume of the halogen based gas are preferably used relative to 100parts by volume of the inert gas. When the halogen based gas iscontained more than 10 parts by volume, the SiC coating is degraded.When it is less than 0.1 part by volume, no effect of adding the halogenbased gas is achieved. In any event, the addition of the halogen basedgas further increases the dehydrating ability of the gas mixture.

A heating temperature at which the dehydrating and purifying treatmentcan be effectively carried out is in the range of 900° to 1200° C. Whenthe heating temperature is lower than 900° C., the dehydration andpurification are insufficient. When it is higher than 1200° C., theporous glass preform contracts whereby diffusion of the dehydrating andpurifying gas into the preform inside and volatilization of impuritiesto be removed from the preform inside are suppressed.

In an alternative manner to prevent the degradation of SiC in thepresence of the halogen based gas contained in the dehydrating andpurifying gas, the porous glass preform is dehydrated and purified in aheating furnace having a quartz made muffle tube firstly, and thenheated in the muffle tube coated with SiC. The quartz made muffle tubeis not degraded by the halogen based gas such as Cl₂, but thetemperature thereof cannot be lowered since crystallization happens at atemperature above 1200° C. as described above. In order to avoid thecrystallization, the dehydrating and purifying treatment in the quartzmade muffle tube is preferably performed at a temperature lower than1100° C.

Next, the second aspect of the present invention is explained.

In a second aspect, the preform is preferably heated at a temperature ofnot higher than 1400° C. A silicon fluoride such as SiF₄, a carbonfluoride such as CF₄ or SF₆ can be used as the fluorine compound gas.

In the case where the porous glass preform is not vitrified by thefluorine doping treatment at a temperature not higher than 1400° C., thepreform is again treated for vitrification at a temperature above 1400°C. under an atmosphere of only the inert gas, or the preform isvitrified while it is fluorine-doped under an atmosphere containing SiF₄together with at least one of Si₂ F₆ and Si₃ F₈ at a temperature above1400° C. In this way, the fluorine-doping treatment can be performed byusing the SiC coated muffle tube without the degradation of the SiClayer.

Basic experiments and considerations to create the second aspect of thepresent invention will be hereinafter described in detail.

EXPERIMENT 4

A sintered SiC article having a diameter of 5 mm was heated at atemperature of 1450° C. under an atmosphere of SiF₄ gas. After 10 hourheating, a weight loss of the SiC article was 2.4%. When other fluorinecompound gas was used instead of SiF₄, the weight loss was almost thesame as in SiF₄.

EXPERIMENT 5

A sintered SiC article having the same size as in Experiment 4 washeated at a temperature of 1400° C. under an atmosphere of SiF₄ gas.After 50 hour heating, no weight loss of the SiC object was measured.When other fluorine gas was used instead of SiF₄, the same results aswith SiF₄ were obtained.

EXPERIMENT 6

A sintered SiC article having the same size as in Experiment 4 washeated at a temperature of 1500° C. under an atmosphere of 91% by volumeof SiF₄ and 9% by volume of Si₂ F₆ for 50 hours, and no weight loss wasdetermined.

EXPERIMENT 7

Experiment 6 was repeated except that the heating temperature was 1650°C. The article was heated for 10 hours and the weight loss thereof was3%.

EXPERIMENT 8

Experiment 7 was repeated except that the gas contained 85% by volume ofSiF₄, 10% by volume of Si₂ F₆ and 5% by volume of Si₃ F₈ was used. After10 hour heating, no weight loss was determined.

In addition, it has been found that Si adheres to a low temperatureportion of the muffle tube when the weight loss of the SiC article wasobserved.

From the results of the above Experiments 4 to 8, the following can beunderstood.

(1) SiC reacts with the fluorine compound gas at a temperature above1400° C. However, by adding Si₂ F₆ as a reactant gas to SiF₄, thereaction is suppressed.

(2) The gas mixture of SiF₄ and Si₂ F₆ reacts with SiC at a further hightemperature, but it does not react when Si₃ F₈ is added to the gasmixture.

(3) The produced material through the reaction between SiC and Si_(y)F_(x) is Si in the low temperature portion.

As a result, the reaction of SiC and the fluorine compound gas issuppressed at a temperature below 1400° C. Even at a temperature higherthan 1400° C., the reaction of SiC with SiF₄ is suppressed by additionof Si₂ F₆ and/or Si₃ F₈. Thus, guidelines on preventing the degradationof SiC are indicated.

The above results can be explained as follows. The degradation reactionof SiC is expressed according to the following reaction equations:

    SiC+3SiF.sub.4 →4SiF.sub.3.↑+C                (2)

    SiC+SiF.sub.4 →2SiF.sub.2.↑+C                 (3)

Free energy changes ΔG in reactions of the above equations (2) and (3)are positive up to 1800° C. and reactions hardly proceed to the righthand side of the equations. However, in a gas flowing system as in amuffle tube of an electrical heating furnace, each equilibrium isshifted and the reaction proceeds to the left hand side a little.Therefore, the SiC coating is degraded a little. When the gas such asthe reaction products SiF₃ and/or SiF₂ are added to the reaction system,the reaction equilibrium is kept and the degradation of the SiC coatingis suppressed.

Si₂ F₆ or Si₃ F₈ forms SiF₃ or SiF₂. at a high temperature according tothe following equations:

    Si.sub.2 F.sub.6 →2SiF.sub.3.                       (4)

    Si.sub.3 F.sub.8 →2SiF.sub.3.+SiF.sub.2.            (5)

The products SiF₃. and SiF₂. suppress the reactions (2) and (3).

The muffle tube preferably used in the present process has an innerlayer made of a highly pure silicon carbide or has an inner and an outerlayers each made of the highly pure silicon carbide.

The highly pure silicon carbide used for the muffle tube has preferablya purity of not less than 99.999% and preferably contains iron of notmore than some ppm and copper of not more than 1 ppm.

The muffle tube in the heating furnace used in the present invention hasthe highly pure silicon carbide layer as its inner layer or as its innerand outer layers. The silicon carbide layer is preferably formed by acoating method with vapor phase reaction (CVD method) such as plasma CVD(PCVD) coating method or chemical CVD coating method since a highly pureand dense coating can be formed by such method.

A thickness of the highly pure silicon carbide coating can be selecteddepending on the operation temperature of the muffle tube and theatmosphere in the muffle tube. Generally, the coating has a thickness ofat least 1 μm, preferably at least 5 μm, more preferably at least 25 μm,for example 50 μm.

Carbon, alumina or SiC sintered material can be exemplified as amaterial for the muffle tube substrate. The carbon material, especiallya highly pure carbon material is preferred.

In the case where the highly pure carbon is used for the muffle tubesubstrate, the impurity of the carbon expressed as a total ash contentis not more than 50 ppm, preferably not more than 20 ppm. For example,when the carbon has the ash content of more than 1000 ppm, it cannot beused for making the muffle tube substrate in view of the impurities suchas iron and copper. The impurities and their amounts contained in thecarbon having the total ash content of not more than 20 ppm are shown inthe following Table:

                  TABLE 1                                                         ______________________________________                                        B < 0.1 ppm         Ca < 0.1 ppm                                              Mg < 0.1 ppm        Ti < 0.1 ppm                                              Al < 0.1 ppm        V < 0.1 ppm                                               Si < 0.8 ppm        Cr < 0.1 ppm                                              P < 0.2 ppm         Fe < 0.1 ppm                                              S < 0.1 ppm         Cu < 0.1 ppm                                              Ni < 0.1 ppm                                                                  ______________________________________                                    

FIG. 2 schematically shows a sectional view of a heating furnace withwhich the present process is performed. In FIG. 2, the reference number21 indicates a porous glass preform, 22 does a supporting rod for thepreform, 23 does a muffle tube, 24 does a heater, 25 does a furnacebody, 26 does an inlet for introducing an inert gas inside the furnacebody and 27 does an inlet for introducing an atmosphere gas (forexample, SF₆, helium and so on) into the muffle tube. The referencenumber 231 indicates a muffle tube substrate made of carbon, 232 does acoating layer of the highly pure silicon carbide. In the embodiment asshown in FIG. 2, the coatings of the highly pure silicon carbide arepresent as an inside layer and an outside layer of the muffle tube 23.

FIG. 3 schematically shows a sectional view of a heating furnace for theoptical fiber preform which is also used in accordance with the presentinvention. In the embodiment shown in FIG. 3, is used a member 31 whichis made of a heat resistant material having a small gas permeabilitysuch as a ceramic or a metal material and in which a muffle tube 23 isinserted in order to prevent penetration of a contaminant into themuffle tube through a wall thereof. At least an inner surface of themuffle tube 23 is coated with the highly pure carbon (not shown).

A material used for the member 31 is preferably the ceramic or the metalmaterial having a nitrogen permeability in the order of 10⁻⁶ cm² /sec.or less. Examples of the ceramic material are, in addition to siliconcarbide, quartz glass, Al₂ O₃, BN and so on.

In another embodiment of an heating furnace which is used in the presentinvention, a muffle tube comprises an upper portion, a middle portionand a lower portion which are detachably connected each other. At leastthe middle portion is made of the highly pure carbon, and the upper andthe lower portion are made of a heat and corrosion resistant material.

One example of the above embodiment is schematically shown in FIG. 4. Aheater 24 is located inside a heating furnace body 25. A muffle tube 23comprises an upper portion 434, a middle portion 435 and a lower portion436 and they are detachably connected each other by, for example,screwing. At least an inner surface of the middle portion 435 is coatedwith a layer 432 made of the highly pure silicon carbide. Since theupper portion 434 and the lower portion 436 are not subjected to so hightemperature as in the middle portion 435, they are not necessarily madeof such highly pure material as in the middle portion 435, but they onlyneed to have the gas impermeabilities. Thus, although the upper and thelower portions are shown to have the layers 432 made of the highly puresilicon carbide as shown in FIG. 4, the layers are preferably made of ausual silicon carbide material from a view point of economy which doesnot have such high purity as the silicon carbide material as describedabove. For example, a silicon carbide material having a purity of 99.9%is satisfactory. In addition, since the upper and lower portions areheated at a temperature not higher than 1000° C., they may be made of aquartz material which is not resistant to a fluorine based gas. Even insuch case, the contents of copper and iron, especially copper contentshould be taken into account and they are preferably not more than 0.1ppm.

The muffle tube having the middle portion at least the inner surface ofwhich is coated with the highly pure silicon carbide is suitably used inthe present invention since it does not react with the halogen based gassuch as SiF₄, Si₂ F₆, CF₄ and C₂ F₆, and it is highly heat resistant.

In the case where the heating furnace as shown in FIG. 2, 3 or 4 isused, a large amount of air around the muffle tube (or environmentaloperation atmosphere) penetrates into the muffle tube, when the glasspreform is inserted in or removed from the muffle tube.

FIG. 5 schematically shows an apparatus which measures an amount of airinflow into a muffle tube comprising the muffle tube 51, an inlet 52 forintroducing a purging gas, a sampling tube 53 for a gas in the muffletube, a device 54 which measures an oxygen concentration in the gas anda pump 55. An inner diameter of the muffle tube 51 is 150 mm and a tipportion of the sampling tube is located at a position by 1 m from anopening of the muffle tube.

Results of the oxygen concentration measurement carried out with respectto one embodiment with the above measuring device are shown in a graphin FIG. 6. As understood from the graph, the air around the muffle tubepenetrates into the muffle tube and it is impossible to prevent theinflow of the air even when an amount of the purging nitrogen isincreased.

The inflow of the air causes problems as follows: Firstly, inside of themuffle tube is contaminated with dusts in the air. The dusts comprisesSiO₂, Al₂ O₃, Na₂ O, Fe₂ O₃ and so on. Among them, Al₂ O₃ and Na₂ Ocause devitrification of the preform, and Fe₂ O₃ causes increase of atransmission loss.

Such inflow of the air is prevented in an additional embodiment of theheating furnace in which the present invention is performed. The heatingfurnace comprises a heater, a muffle tube and a front chamber whichpreviously or subsequently accommodates the glass preform before orafter a thermal treatment of the preform, and from or in which thepreform is inserted in or removed from the muffle tube.

FIG. 7 schematically shows a heating furnace which is the same as thatshown in FIG. 4 except that it comprises the front chamber 71. When thefurnace is used, the glass preform 21 to be thermally treated isinserted in the front chamber 71 and a top cover (not shown) of thefront chamber is closed. Then, nitrogen is supplied in the front chamberwhich is separated from the muffle tube 23 with a partition means 76 fornitrogen replacement. The partition means 76 is opened, the porous glasspreform is inserted in the muffle tube 23 of which inside atmosphere hasbeen previously replaced with a desired atmosphere gas, the partitionmeans is closed and then the thermal treatment is initiated. After thethermal treatment, the partition means is opened, the partition means isclosed after the glass preform is lifted from the muffle tube into thefront chamber, and then the glass preform is removed from the frontchamber after opening the top cover.

The front chamber is preferably constructed so that it can be evacuatedto a pressure of 10⁻² Torr and be heated to a temperature of 800° C. Thefront chamber is preferably made of a material which is heat resistantand which does not liberate any contaminant. For example, quartz glass,SiC, Si₃ N₄ or BN is preferred for making the front chamber. Thematerial of the chamber may the same as or different from that of themuffle tube.

When the front chamber is evacuated, for example a rotary pump may beused. In order to prevent a back flow of a pump oil, a trap cooled withliquid nitrogen may be located between the front chamber and the pump. Arotary installing mechanism having a magnetic seal is disposed at thetop of the front chamber.

EXAMPLES

The present invention will be further described with the followingexamples.

EXAMPLE 1

A heating furnace comprising a muffle tube 23 having a SiC coating 232in a thickness of 50 μm as shown in FIG. 2 was used. A porous glasspreform 21 was inserted in the muffle tube at a temperature of 1100° C.5 l/min. of He and 300 cc/min. of SiCl₄ were supplied in the muffle tubeas the dehydrating and purifying gas. The preform was lowered and passedalong a side portion of a heater 24 through the muffle tube at alowering rate of 5 mm/min. and thereby the preform was dehydrated.Subsequently, the temperature of the furnace was raised to 1650° C. andthe preform was again passed along the side portion of the heater at arate of 2 mm/min. while supplying He at 5 l/min., whereby the preformwas vitrified.

An optical fiber was drawn from the glass preform obtained by the abovetreatment. A residual water content in the optical fiber was less than10 ppb to show that substantially no water was present in the opticalfiber. A transmission loss was measured and the results thereof showedno absorption increase of the optical fiber due to a transition metalsuch as Fe, Cu, Cr or Ni (see FIG. 8, curve I).

The above treatment was carried out to vitrify thirty preforms. Afterthe thirtieth treatment, a surface of the muffle tube was observed. Nodegradation of the SiC coating was observed.

EXAMPLE 2

Example 1 was repeated except that a gas mixture consisting of 5 l/min.of He, 200 cc/min. of SiCl₄ and 50 cc/min. of Cl₂ was used as thedehydrating and purifying gas.

The surface of the muffle tube was observed after treating thirtypreforms. No degradation of the SiC coating was observed as in Example1.

COMPARATIVE EXAMPLE 1

Example 1 was repeated except that 5 l/min. of He and 200 cc/min. of Cl₂were supplied as the dehydrating and purifying gas.

The treatment was carried out on ten preforms and an optical fiber wasdrawn from each preform. The water content of each optical fiber wasdetermined and was as large as 0.2 to 1 ppm in the optical fibers drawnfrom the third to tenth preforms. The wavelength transmission losscharacteristic of each optical fiber was measured and showed not only anabsorption peak due to OH at 1.4 μm but also an absorption peak of 3 to10 dB/km due to Cu²⁺ at 0.85 μm (see FIG. 8, Curve II).

The surface of the muffle tube was observed after the treatment of thetenth preform. The SiC coating in a heating portion of the muffle tubewas completely changed and green crystal of CuCl₂ was deposited onelectrodes (made of copper) of the heater in the furnace body due tocorrosion with Cl₂.

EXAMPLE 3

According to the present invention, a dehydrated porous glass preformwas fluorine-doped in a heating apparatus as shown in FIG. 2.

The preform formed by VAD had been previously dehydrated in a quartzmade muffle tube and contained SiO₂ as a main component. The preform hada diameter of 140 mm and a length of 500 mm. Details of this examplewere as follows:

The porous preform was inserted in the quartz made muffle tube at atemperature of 1000° C. under an atmosphere of He containing 5% of Cl₂at a lowering rate of 5 mm/min. for dehydration. After the dehydration,the diameter and the length of the preform were the same as those beforethe dehydration.

The porous preform dehydrated as in the above was inserted in a muffletube made of the highly pure carbon having the coating of SiC. Thefluorine doping was carried out at a temperature of 1370° C. under anatmosphere of He containing 3% of SiF₄ at a lowering rate of 3 mm/min.Then, the temperature was raised to 1600° C. and the preform wasvitrified under an atmosphere of He. The obtained preform had a diameterof 60 mm and a length of 300 mm. A refractive index of the glass preformwas -0.34% expressed in terms of a specific refractive index differencefrom quartz glass.

A silica core single mode optical fiber was produced by using thepreform and the transmission loss thereof was determined. The losseswere 0.31 dB/km and 0.17 dB/km at wavelengths of 1.3 μm and 1.55 μm,respectively. No impurity was found and also no abnormal peak wasobserved after H₂ test (at 100° C. for 20 hours). Although temperatureraising and lowering operations of the quartz made muffle tube wererepeated, the muffle tube was not destroyed.

COMPARATIVE EXAMPLE 2

Example 3 was repeated to fluorine-dope of the porous preform which hadbeen dehydrated in the quartz made muffle tube and to produce an opticalfiber except that a muffle tube was made of a quartz glass containing 1ppm of copper and comprised no carbon coating.

The residual water content in the produced fiber was 0.01 ppm. Anabsorption due to copper was present to about 1.30 μm, which wassufficiently small in comparison with that found in an optical fiberproduced with the prior art technique. The absorption was 2 to 3 dB/kmat a wavelength of 0.8 μm. However, an inner wall of the muffle tube washeavily etched, and the muffle tube had problematic corrosionresistance.

COMPARATIVE EXAMPLE 3 Heat Resistance of Quartz Made Muffle Tube

Example 3 was carried out repeatedly except that a quartz made muffletube was used instead of the carbon made one. The muffle tube was highlystretched during the vitrification treatment and it could not be usedagain.

COMPARATIVE EXAMPLE 4 Etching of Quartz Made Muffle Tube

Comparative Example 3 was repeated except that SF₆ was used instead ofSiF₄. The quartz made muffle tube was heavily etched and pin holes werefound in a wall of the muffle tube near the heater. The glass preformcontained water as much as in the order of some ppm. The stretch of themuffle tube was of course in remarkable extent and the muffle tube couldnot be used again.

EXAMPLE 4

Twenty glass preforms for an optical fiber were treated by using aheating furnace as shown in FIG. 2. The furnace comprised a carbon mademuffle tube having the SiC coating in a thickness of 60 μm. Theatmosphere of the treatment was SiF₄ /Si₂ F₆ /He=3%/0.3%/96.7% and theheating temperature was 1650° C. The obtained preform had a specificrefractive index difference of -0.3% relative to quartz. After thetreatment of the preforms, no degradation of the SiC coating wasobserved on the surface of the muffle tube.

EXAMPLE 5

By using an apparatus as in Example 4, twenty porous fluorine-dopedglass preforms was vitrified under an atmosphere of SiF₄ /Si₂ F₆ /Si₃ F₈/He=3%/0.3%/0.1%/96.6% at a temperature of 1650° C. After the vitrifyingtreatment, the surface of the muffle tube was observed and no corrosionof the SiC coating was found.

A single mode optical fiber was produced from the obtained preform. Thetransmission losses of the optical fiber were 0.35 dB/km and 0.20 dB/kmat wavelengths of 1.3 μm and 1.5 μm, respectively.

EXAMPLE 6

By using an apparatus as in Example 4, fifteen fluorine-doped glasspreforms were vitrified under an atmosphere of SiF₄ /Si₃ F₈/He=3%/0.1%/96.9% at a temperature of 1650° C. After the vitrifyingtreatment, the surface of the muffle tube was observed and no corrosionof the SiC coating was found.

A single mode optical fiber was produced from the obtained preform. Thetransmission losses of the optical fiber were 0.33 dB/km and 0.20 dB/kmat wavelengths of 1.3 μm and 1.5 μm (not FIG. 8 but just like (quitesimilar to) FIG. 8), respectively.

COMPARATIVE EXAMPLE 5

When a carbon made muffle tube having a SiC coating in a thickness of 60μm in a heating apparatus as shown in FIG. 2 was kept at a temperatureof 1550° C. under an atmosphere of He containing 5% of SiF₄ for tenhours, the SiC coating was fully volatilized.

EXAMPLE 7

A carbon made muffle tube having an inner coating of the highly puresilicon carbide in a thickness of 50 μm formed by the CVD method washeated at a temperature of 1400° C. under an atmosphere of 160 ml/min.of SiF₄ and 10 l/min. of He. A porous glass preform was inserted in themuffle tube at a lowering rate of 3 mm/min. After the preform was passedalong a side portion of the heater, the atmosphere gas was changed to 10l/min. of He and the heater temperature was raised to 1650° C. Thepreform was again passed along the side portion of the heater at a rateof 15 mm/min.

The obtained preform had a refractive index difference of -0.33%relative to quartz. A single mode optical fiber having a diameter of 125μm was produced from the preform. The residual water content of theoptical fiber was 0.02 ppm and no increase of the transmission loss dueto, for example, copper and iron was observed.

By using the same muffle tube, fifty glass preforms were treated.Although the inner SiC coating was reduced by 5 μm in a thickness, noincrease of the transmission loss was observed.

COMPARATIVE EXAMPLE 6

Example 7 was repeated except that the carbon made muffle tube did nothave the highly pure silicon carbide coating. SiF₄ highly leaked througha wall of the muffle to the outside and a concentration of HF near themuffle tube was increased to 1 to 5 ppm. The obtained preform had awater content of 1.5 ppm.

EXAMPLE 8

A porous glass preform was purified in a quartz made muffle tube at atemperature of 1100° C. under an atmosphere of Cl₂ /He=0.3/10 by volume.Then, the preform was vitrified at a temperature of 1350° C. under anatmosphere of 100% of SiF₄ and then at a temperature of 1500° C. and atan atmosphere of 100% of He in a muffle tube having an inner SiC coatingin thickness of 50 μm formed by the CVD method. The obtained preform hada specific refractive index difference of -0.68% relative to quartz.

A pure silica core single mode optical fiber having a diameter of 125 μmwas produced from the preform. The transmission loss of the opticalfiber was 0.23 dB/km at a wavelength of 1.55 μm.

EXAMPLE 9

An apparatus comprising a carbon made muffle tube as shown in FIG. 7 wasused. The muffle tube had silicon carbide coatings (purity 100%) formedby the CVD method on an inner and an outer surfaces. A glass preform 21was inserted in the front chamber 71, the top cover was closed and thefront chamber interior was replaced with nitrogen gas. Then, the preformwas inserted in the muffle tube 23 after the partition means 76 wasopened, and the partition means was closed. The preform wasfluorine-doped as in Example 7.

When the preform was removed from the muffle tube, following procedureswere carried out: The muffle tube interior was replaced with N₂ gas, thepartition means was opened, the preform was lifted in the front chamber,the partition means was closed, and then the top cover was opened toremove the preform from the front chamber.

Seventy preforms were treated in accordance with the above procedures inone and a half months. A silica core single mode optical fiber having adiameter of 125 μm was produced from the obtained preform. An averagetransmission loss of the optical fiber was 0.180 dB/km at a wavelengthof 1.55 μm.

COMPARATIVE EXAMPLE 7

By using a heating furnace as shown in FIG. 2, forty-five preforms werethermally treated in one month. The muffle tube had an outer and aninner coatings of silicon carbide each having a thickness of 50 μmformed by the CVD method (purity 100%). The preform was treated at atemperature of 1350° C. under an atmosphere of a SiF₄ containing gas.

A silica core single mode fiber was produced from each obtained preform.An average transmission loss of the optical fibers produced from thefirst preform to the thirtieth preform was 0.183 dB/km at a wavelengthof 1.55 μm. An average transmission loss of the optical fibers producedfrom the thirty-first preform to the last preform was as rather large as0.195 dB/km at a wavelength of 1.55 μm.

A sodium content in the preform was measured by atomic-absorptionspectroscopy. The Na content was less than 20 ppb by weight in the tenthpreform, and 150 to 200 ppb by weight in the last preform. This may bebecause of entry of dusts in the air around the muffle tube.

Therefore, when the apparatus comprising the front chamber is used, thebetter preform can be stably formed for a long period.

EXAMPLE 10

A heating furnace as shown in FIG. 2 was used. The muffle tube in thefurnace had an outer and an inner coatings of silicon carbide (purity100%) formed by the CVD method each having a thickness of 50 μm. Aporous glass preform was heated at a temperature of 1050° C. under anatmosphere gas of 300 cc/min. of SiCl₄ and 10 l/min. of He at a loweringrate of 10 mm/min. After the preform was passed along a side portion ofthe heater, the atmosphere gas was changed to 160 cc/min. of SiF₄ and 10l/min. of He and the heater temperature was raised to 1400° C. Thepreform was again passed along the side portion of the heater at a rateof 3 mm/min. Then, the gas was changed to 10 1/min. of He and the heatertemperature was raised to 1700° C. The preform was moved at a rate of 20mm/min. to be vitrified.

The obtained preform had a specific refractive index difference of-0.32% relative to quartz. A pure silica core single mode optical fiberhaving a diameter of 125 μm was produced from the preform. Thetransmission loss of the fiber was 0.178 dB/km at a wavelength of 1.55μm.

EXAMPLE 11

By using the same apparatus as in Example 10, a glass preform consistingof a silica core containing 10% of GeO₂ and a cladding of pure silicawas thermally treated for dehydration and vitrification.

An optical fiber having a diameter of 125 μm was produced from theobtained preform. The transmission loss of the fiber was 0.35 dB/km at awavelength of 1.3 μm.

EXAMPLE 12

In a heating furnace as shown in FIG. 3, a carbon tube 31 having asilicon carbide coating (purity 100%) on its surface was used as aninserted member in a muffle tube 23. The coating was 50 μm in thicknessand formed by the CVD method. An impurity content of the member was lessthan 5 ppm by weight.

SiF₄ and He were supplied in the muffle tube at rates of 160 cc/min. and10 l/min., respectively. 20 l/min. of N₂ was introduced in the annularspace between the muffle tube and the member. The muffle tube was heatedto a temperature of 1400° C. and a porous preform 21 was passed along aside portion of the heater 24 at a lowering rate of 3 mm/min. to befluorine-doped.

Then, the muffle tube was heated to a temperature of 1650° C. and thegas to be supplied in the muffle tube was changed to 10 l/min. of He.The gas to be supplied in the annular space remained unchanged. Thepreform was again passed along the side portion of the heater at alowering rate of 15 mm/min. to be vitrified.

A pure silica core single mode optical fiber having a diameter of 125 μmwas produced from the preform. The transmission loss of the fiber was0.178 to 0.181 dB/km at a wavelength of 1.55 μm.

EXAMPLE 13

A heating furnace as shown in FIG. 2 was used. A muffle tube in thefurnace had an outer and an inner coatings of the high purity siliconcarbide (purity 100%) formed by the CVD method each having a thicknessof 50 μm. As in Example 12, seventy preforms were treated.

A pure silica core single mode optical fiber having a diameter of 125 μmwas produced from each preform. The transmission loss of the opticalfiber highly scattered in the range of 0.182 to 0.195 dB/km at a wavelength of 1.55 μm.

As explained with reference to Examples and Comparative Examples, whenthe muffle tube having the coating of SiC is used in accordance with thepresent process, the preform for the optical fiber having substantiallyno water and substantially no impurity can be stably formed so that theoptical fiber having a less transmission loss can be drawn from suchpreform.

In addition, since the muffle tube does not destroy due to thetemperature lowering operation in comparison with the conventionalquartz made muffle tube, the muffle tube can be economically and stablyused for a long period.

We claim:
 1. A heating process for the dehydrating and purifying of aporous glass preform for an optical fiber comprising passing the porousglass preform through a muffle tube at a high temperature in anatmosphere comprising an inert gas and a silicon tetrachloride, whereinsaid tube comprises at least one of carbon, alumina, and SiC sinteredmaterial and said tube forms a cavity having an opening at one endthrough which said porous glass preform passes into said tube and thesurface of the tube exposed to the silicon tetrachloride is a SiC layer.2. The process according to claim 1, wherein the atmosphere furthercomprises a chlorine gas.
 3. The process according to claim 2, wherein avolume percentage ratio of the chlorine gas to the inert gas is from 0.1to 10 percent and a volume percentage ratio of the silicon tetrachloridegas to the inert gas is from 0.3 to 10 percent.
 4. The process accordingto claim 1, wherein a volume percentage ratio of the silicontetrachloride to the inert gas is from 0.3 to 10 percent.
 5. The processaccording to claim 1, wherein a heating temperature of the preform is inthe range of 800° to 1200° C.
 6. The process according to claim 1,wherein the SiC layer has a purity expressed in terms of a total ashcontent of not more than 50 ppm.
 7. The process according to claim 1,wherein the SiC layer is formed with a method selected from the groupconsisting of PCVD coating method and CVD coating method.
 8. The processaccording to claim 1, the SiC layer has a thickness of not less than 1μm.